Known methods of banknote thickness gauging are based on mechanic contact with the banknotes. These methods limit the speed of the banknote transport through the gauge.
Opto-electronic devices based on the reflection of light are known for non-contact measuring the variations in height on one side of a banknote, but not for gauging the thickness of the banknote.
Apparatuses of this kind, besides, involve disadvantages in the form of high costs due to high requirements on the precision of prisms and the like, and of a substantial maintenance demand.
The gauging result, moreover, can be unreliable, because the light beam reflected from black-print portions is deformed.
Apparatuses for non-contact gauging the thickness of strips are previously known. At some of these apparatuses each gauge consists of a nozzle, by which air is ejected against the object, and a measure of the distance between the nozzle and the object is obtained which is in the form of an air pressure. The variations in thickness are recorded as differences in pressure. Apparatuses of this kind have disadvantages which are connected with the compressibility of the air and the normally small magnitude of the pressure differences to be measured. These disadvantages express themselves in the form of reduced accuracy.